1. Field of the Invention
This invention relates to a new process for the preparation of (cyclo)aliphatic diisocyanates by the phosgenation of (cyclo)aliphatic diamines in the gas phase.
2. Description of the Invention
The preparation of isocyanates by the reaction of amines with phosgene in the gas phase has been known for a long time (see Siefken, Annalen 562, 108 (1949)), but gas phase phosgenation has hitherto achieved technical importance only for the reaction of monoamines because polyfunctional amines partly decompose on evaporation and tend to form polymers during phosgenation (see Ullmann, 4th Edition, Vol. 13, page 353), so that the yields obtained are generally low.
A process in which hexamethylenediamine is reacted with phosgene in the gas phase in the presence of nitrogen and benzene to form hexamethylene diisocyanate is described in DE-PS No. 870 847. The yield obtained is approximately 10 to 15% of the theoretical yield. This unsatisfactory yield is a disadvantage of the process.
Two publications (J. Chem. Soc. Japan, Ind. Chem. Sect. 55, 266-7 (1952) and Ibid. 56, 289-290 (1953)) describe the gas phase synthesis of isocyanates in which the amine evaporates off together with toluene and is reacted in the gas phase with a stoichiometric excess of phosgene, amounting to 130% of the theoretical quantity, in a tube reactor at temperatures of at most 325.degree. C. The isocyanate in vapor form is condensed in toluene, but parts of the isocyanate react with hydrogen chloride to form solid carbamic acid chloride in the process. To convert the carbamic acid chloride into the desired isocyanate, the suspension of carbamic acid chloride in toluene is heated under reflux until no more hydrogen chloride evolves. The reaction of hexamethylenediamine by this process at 280.degree. C. to 300.degree. C. results in a yield of 80%. The disadvantages of this process lie in the low yield and the cost intensive thermal decomposition of the carbamic acid chloride. Both these disadvantages would appear to render the process uneconomical.
GB-P No. 1 165 831 describes inter alia a process for the gas phase phosgenation of diisocyanates in which the reaction of the amine in vapor form with phosgene is carried out at temperatures of 150.degree. to 300.degree. C. in a tube reactor equipped with mechanical stirrer. The reactor resembles a thin layer evaporator in which the stirrer mixes the gases and at the same time sweeps over the heated walls of the tube reactor to prevent a build-up of polymer material on the wall of the tube. Solid substances would reduce the transfer of heat and cause the reaction to stop. The use of a stirrer rotating at about 1000 revs/min in combination with phosgene at a temperature of about 300.degree. C., however, requires considerable safety precautions to seal off the reactor and to run the stirrer in bearings in the highly corrosive medium.
It has now surprisingly been found that (cyclo)aliphatic diisocyanates may also be prepared in high yield by gas phase phosgenation of the precursor diamines without the disadvantages of the known art processes if certain process parameters to be described in more detail below are observed during the reaction and the gas mixture formed in the reaction is processed by the method also described below.